i used to send cheap ones in my care packages
you can get them very cheap if you find the right supplier..but they arent “official”
example
https://www.medalcraft.com/product-category/challenge-coins/
btw, in return for one of the packages I got a nice letter from a commander at the base and an “official pin” worn on the uniforms belonging to the forces helping..and i surely cant remember the details on it. I will have to dig that back up sometime. seems like it was Iraq but might be wrong